Printing machines for printing individual sheets can print sheets of different materials and of different sizes. Such printing machines for printing individual sheets comprise, as a rule upstream of at least one printing unit, a sheet-alignment unit. This sheet-alignment unit serves to exactly align supplied sheets with respect to the sheet transport path through the printing unit, before the sheets are passed on for printing or further processing. To accomplish this, the position of the supplied sheets is detected by sensors and forwarded to a control device. Usually, the control device controls various alignment rollers that align a misaligned sheet relative to the sheet transport path.
Known sheet-positioning arrangements comprise, for example, adjacent axially shiftable pairs of rollers that are capable of shifting a supplied sheet in a direction transverse to the sheet transport path (crosstrack alignment). Furthermore, sheet-alignment arrangements are known that comprise pairs of rollers transverse to the sheet transport path at a distance from each other, which pairs of rollers can be driven in different ways in order to align the lead edge of a supplied sheet in a direction perpendicular to the sheet transport path, before said sheets pass through the printing machine (angle or skew alignment).
The accuracy in determining and correcting a misalignment of a sheet depends on the distance of the alignment rollers from each other and on the sensors that are being used (skew sensors that measure the incoming lead edge of a sheet). For a rigid installation, the distance of the alignment rollers is determined by the smallest sheet width that is to be processed by the printing machine. The resolution of a potential angle adjustment of the sheet is highly dependent on the distance between the alignment rollers. At a small distance of the alignment rollers and with large sheet formats, an accurate angle adjustment of the sheet is not possible. A small distance of the alignment rollers primarily restricts the accuracy of the angle adjustment process. Therefore, it is desirable to adapt the distance between the alignment rollers to the width of the supplied sheet.
In most instances, known sheet-positioning and sheet-alignment arrangements are complicated because they, on the one hand, comprise pairs of rollers for positioning the sheet in a direction transverse to the sheet transport path through the machine and, on the other hand, comprise pairs of sheet alignment rollers for aligning the lead edge of a sheet in a direction perpendicular to the sheet transport path. EP 0 469 866 A also discloses an arrangement that performs a skew alignment and a crosstrack alignment by way of the same pairs of rollers. However, the moved masses are very large in crosstrack alignment. This is disadvantageous for a rapid adjustment of the pairs of rollers.
Other known arrangements have been disclosed in U.S. Pat. No. 5,322,273 and in publication DE 101 60 382. U.S. Pat. No. 5,322,273 discloses a complex device that, among other things, is suitable for a sheet alignment perpendicular to the sheet transport path (de-skewing). The rollers or segmented cylinders provided therefor comprise appropriate drives. DE 101 60 382 discloses driven support rollers that can be shifted in transverse direction by a spindle mechanism that is located on the inside.